1. Field of the Invention
The present invention relates to a fixing device and an image formation apparatus. In particular, the present invention relates to technology used in a fixing device comprising a guide plate that guides an induction-heated belt in its rotation direction to suppress the guide plate from generating heat and improve heating efficiency of the belt.
2. Description of Related Art
In recent years, image formation apparatuses (e.g., printers) are starting to incorporate an energy-saving fixing device of an electromagnetic induction-heating type, rather than a fixing device using a halogen heater as a heat source (Japanese Laid-Open Patent Application No. 2007-264421).
FIG. 14 is a cross-sectional view showing the structure of a fixing device 300 of an electromagnetic induction-heating type.
As shown in FIG. 14, the fixing device 300 is composed of: a fixing belt 301; a fixing roller 302; a pressure roller 303; a magnetic flux generator 304; a guide plate 305; and so on.
The fixing belt 301 is a cylindrical, elastically deformable belt comprising an induction-heated layer 301a and a magnetic shunt alloy layer 301b that is provided on the back of the induction-heated layer 301a. The fixing belt 301 is driven and rotated in the direction of arrow P.
The magnetic shunt alloy layer 301b has the property that it is ferromagnetic at ambient temperature, but turns nonmagnetic at temperatures above the Curie temperature.
The fixing roller 302 is positioned inside the rotation path of the fixing belt 301. The pressure roller 303 is positioned outside the rotation path of the fixing belt 301. A fixing nip 310 is formed by the pressure roller 303 pressing the fixing roller 302 with the fixing belt 301 in between. The pressure roller 303 rotates in the direction of arrow Q by receiving a driving force from a driving motor (not illustrated). The fixing roller 302 and the fixing belt 301 are driven and rotated due to this driving force acting thereon.
The magnetic flux generator 304 is positioned outside the rotation path of the fixing belt 301, in such a manner that the fixing belt 301 is positioned between the magnetic flux generator 304 and the pressure roller 303. The magnetic flux generator 304 generates magnetic flux for causing the induction-heated layer 301a of the fixing belt 301 to generate heat.
The guide plate 305 is a nonmagnetic member made from a low-resistance and electrically conductive material. The guide plate 305 is positioned inside the rotation path of the fixing belt 301, in such a manner that the guide plate 305 faces the magnetic flux generator 304 with the fixing belt 301 in between. The guide plate 305 is curved along the curvature of the fixing belt 301. The guide plate 305 controls relative positions of the fixing belt 301 and the magnetic flux generator 304, while guiding the fixing belt 301 in its rotation direction by the surface of the guide plate 305 coming into contact with the inner surface of the rotating fixing belt 301.
In the fixing device 300 configured in the above manner, once the magnetic flux generator 304 starts generating the magnetic flux during the driving/rotation of the fixing belt 301, heat is generated mainly in a portion of the induction-heated layer 301a of the fixing belt 301, the portion facing the magnetic flux generator 304. Once this heat-generating portion of the induction-heated layer 301a reaches the fixing nip 310, the temperature of and in the vicinity of the fixing nip 310 is increased to a temperature suited for the fixing. Then, when toner images formed on a sheet S pass through the fixing nip 310, the toner images are thermally fixed onto the sheet S by thermocompression.
At this time, the temperature of a central portion of the fixing belt 301 that comes in contact with the sheet S is lowered, as the sheet S draws heat from the central portion of the fixing belt 301; however, the temperature of both edges of the fixing belt 301 that do not come in contact with the sheet S (hereinafter, “contactless portions of the fixing belt 301”) remains high, as the heat thereof is not drawn by the sheet S. In such a situation, if power is supplied to the magnetic flux generator 304 so as to set the central portion of the fixing belt 301 at a target temperature, the temperature of the contactless portions will further increase.
If portions of the magnetic shunt alloy layer 301b corresponding to the contactless portions of the fixing belt 301 (hereinafter, “contactless portions of the magnetic shunt allow layer 301b”) are heated to the point where the temperature thereof exceeds the Curie temperature, the contactless portions of the magnetic shunt alloy layer 301b turns from ferromagnetic to nonmagnetic. As a result, the magnetic flux, which had been carried along the magnetic shunt alloy layer 301b, penetrates through the magnetic shunt alloy layer 301b and breaks into the guide plate 305.
As the guide plate 305 is made from a low-resistance and electrically conductive material, an eddy current produced by the magnetic flux that is breaking into the guide plate 305 contributes to generation of magnetic flux whose direction cancels out the magnetic flux that is breaking into the guide plate 305, rather than to generation of heat. Consequently, the magnetic flux density in the contactless portions of the fixing belt 301 is reduced, thus alleviating temperature increase therein.
As set forth above, the fixing device 300 has excellent thermal efficiency because the fixing belt 301 itself generates heat. Moreover, due to the interaction between the magnetic shunt alloy layer 301b and the guide plate 305, the fixing device 300 can automatically perform temperature control so as not to overheat the contactless portions of the fixing belt 301.
However, even though the guide plate 305 is made from a low-resistance and electrically conductive material, it is still unavoidable that the eddy current generates heat. Furthermore, because the guide plate 305 is thin (i.e., has a thickness of approximately 0.5 mm), if the fixing device 300 continuously executes the fixing for small-sized sheets for a long period of time, the amount of said heat will be accumulated and the temperature of the guide plate 305 will be excessively increased. This may thermally deform the guide plate 305.
One way to avoid this problem is to raise heat capacity of the guide plate 305 by increasing the thickness of the guide plate 305. This, however, raises an amount of heat that the fixing belt 301 draws from the guide plate 305 because they are in contact with each other, and therefore creates another problem where it takes time to complete a warm-up.
Furthermore, although the above fixing device of the electromagnetic induction-heating type has an excellent heating efficiency due to the fixing belt 301 generating heat on its own by induction heating, further improvements in heating efficiency has been demanded with the current trend of energy conservation.